Try deleting the problematic file (electing to remove original file if/when prompted) and then re-downloading the file from the iTunes store.
You can re-download content purchased from the iTunes store (availability varies depending on location) using the purchased option from the Quick Links section in the top right corner of the iTunes homepage in your iTunes application on your computer.
You can re-download content purchased from the iTunes store (availability varies depending on location) using the purchased option which is revealed in the iTunes app on your iOS device by tapping the more button at the bottom of the screen.
If the problem re-occurs, select the content which is causing a problem and use the 'Report a problem' button in Your Purchase History using your computer.
I have purchased over 4,000 items from the iTunes Store, I have had issues with 2 of them, they turned out to be a problem with the original encoding and not the download procedure. If you are experiencing this many problems perhaps your issue is with your LAN or your ISP, I find it rather hard to believe these problems are at Apple's end of things and not yours.